In the transmission of digitized speech signals, the transmission bit rate depends on the sampling frequency and the number of bits used to quantize a speech sample. The sampling frequency is constrained by the speech bandwidth. Therefore, the bit rate can be altered only by altering the quantization precision which is directly proportional to the number of bits per sample. The feedback or as it is sometimes termed, the backward-type codec is one type of speech coding configuration available for achieving variable bit rate speech transmission. Such a backward-type codec may be established using an ADPCM codec, variable bit rate being achieved through the provision of multiple quantizers, each producing a digitized sample of a different number of bits. For example, with speech sampled at the rate of 8,000 samples/sec., transmission bit rates of 24, 32 or 40 kbits/sec. are realized with a three quantizer ADPCM system that uses a 3-bit quantizer, a 4-bit quantizer and a 5-bit quantizer.
The backward-type codec, and particulary the ADPCM codec, has advantages over the feed-forward type codec which exhibits considerable processing delay introduced by the block processing nature of a feed-forward configuration and which requires a portion of the available bit rate for transmission of the overhead information associated with each block of speech. While the backward-type codec eliminates these two drawbacks of the feedforward type codec, it suffers the disadvantage that distortion is introduced whenever the bit rate is changed, that is, whenever the codec switches from one quantizer to another. It has been determined that this distortion occurs because of the finite duration required by the quantizer stepsize to track the input signal.